[FFmpeg-devel] [PATCH] ALSA for libavdevice

Michael Niedermayer michaelni
Mon Dec 29 13:40:10 CET 2008

On Mon, Dec 29, 2008 at 10:03:16AM +0100, Luca Abeni wrote:
> Hi Michael,
> On Mon, 2008-12-29 at 04:03 +0100, Michael Niedermayer wrote:
> [...]
> > I think you are missing a small detail here, V4L and OSS support in ffmpeg
> > currently IS broken, and can under some circumstances generate invalid
> > files without the users knowledge. (invalid in the sense of flaky timestamps)
> Well, I do not know what changed in the last two years, but I know for
> sure that about 2 years ago ffmpeg was able to capture perfectly
> synchronised audio and video using v4l2+oss (the result was ok for
> professional broadcasters). The only cases in which A/V desynch was
> noticed, it was due to drivers' bugs, and changing the capture card
> fixed the issue. And I think that if timestamps are not correct/precise
> enough then A/V synch will result incorrect, right?

not neccessarily
using for example -async <some large number> would turn "noisy" timestamps
to audible artifacts but should not lead to AV desync.

the reason why it works with -async 0 is because of a bug, more
precissely, sync_opts is not passed into audio encoders thus wiping out
the wrong timestamps from the audio input.
Once this is fixed OSS/V4L1 will likely stop working reliably unless they
are fixed first.

Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Frequently ignored awnser#1 FFmpeg bugs should be sent to our bugtracker. User
questions about the command line tools should be sent to the ffmpeg-user ML.
And questions about how to use libav* should be sent to the libav-user ML.
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